1. Field of the Invention
The present invention relates to a method of and a device for measuring in two or three dimensions and without contact the position accuracy of mechanical components after assembly such as, for example, mechanisms of drums, posts and the like of video tape recorders (hereafter abbreviated as VTRs).
2. Description of Related Art
In recent years, in VTRs, efforts have been made to improve the accuracy of mechanisms for high-density recording. Also, in order to achieve compatibility between systems, the technology for the high-accuracy measurement of the position of components becomes important to minimize variations between decks.
Explained below is a conventional method of measuring the position accuracy of components related to VTRs after assembly.
First, the mechanism of a VTR to be measured will be explained. A rotary drum performs recording or playback by means of a magnetic rotary head mounted at the bottom end thereof, with a magnetic tape, supplied from a cassette, wound diagonally around the rotary drum. The magnetic tape is not to be run during the measurement. A stationary drum has a stepped or shoulder portion known as a lead which regulates the bottom edge of the magnetic tape. The rotary drum and the stationary drum constitute a drum unit. A roller post positioned upstream of the drum unit with respect to the direction of motion of the magnetic tape stands vertically and regulates the upper-edge of the magnetic tape in motion, and an inclined post changes the direction of motion of the magnetic tape. These posts are hereinafter referred to as upstream posts. An inclined post positioned downstream of the drum unit with respect to the direction of motion of the magnetic tape restores the motion of the magnetic tape to its original direction, and a roller post stands vertically and regulates the upper edge of the magnetic tape. These posts are hereinafter referred to as downstream posts. Thus, the measurement object (object to be measured) in this example comprises the drum unit, roller posts, and inclined posts, all of which are regarded as the main components of the VTR mechanism.
Next, a conventional, contact-type device for measuring position coordinates will be explained. First, a measurement object is set on a reference plane of a measurement platform. A probe which is affixed to the contact-type device for measuring position coordinates is capable of moving in X, Y and Z directions. Also, each of the X, Y and Z coordinates is detected by a position coordinate detection device. A computation device computes the position coordinates in X, Y and Z directions from the values that have been detected by the position coordinate detection device, and computes the angle of inclination and the direction of inclination of the measurement object and the distance between the centers of the posts.
With regard to the conventional contact-type position coordinate measurement device configured in the above manner, its measurement method will be explained below.
First, the method of measuring the upstream inclined post will be explained. Set the reference directions, X and Y, and set, for instance, the center of the moving magnetic tape as the reference height of the mechanism. Next, at an arbitrary height, let the probe contact the outer periphery of the inclined post at a minimum of 3 points, find the X, Y coordinates, and from those values find the center of the post. Similarly, take measurements at a different height, and find the center of the post. In effect, by taking measurements at arbitrary heights at a minimum of two places, a center line connecting the coordinates of the centers of the inclined post can be found. Similarly, a center line of the upstream roller post can be found.
In this way, all of the inclination angles, inclination directions and x, y coordinates at arbitrary heights of the inclined posts, and the distance between the centers of the posts at the reference height can be found by computation with the computation device.
However, as explained below, there are problems with the aforementioned conventional setup:
(1) Since the rotary drum is a rotating body, it is difficult to bring the probe into contact therewith when the rotary drum is rotating. Accordingly, no measurement can be made during rotation and, hence, no measurement can be made of the relative position between the drum unit and other posts, which is the most important measurement.
(2) Since the inclined post is inclined at an angle, the probe can be brought into contact with only a limited portion thereof in obtaining the center thereof. Hence, errors are likely to occur when the center of the circle is obtained. Furthermore, the larger the angle of inclination, the larger this trend becomes. Also, the distance between the inclined post and the roller post is very narrow, and is expected to become even narrower as miniaturization of the mechanism continues further in the future, so that the measurement portion will be further restricted.
(3) A post that is small in diameter and low in stiffness will change its shape when a load is applied due to probe contact, and therefore accurate measurement cannot be made.
(4) Contact-type measurement cannot be made when a magnetic tape is wound around the measurement object. However, when recording or playback is actually done, a force is applied to the measurement object, since the magnetic tape is wound therearound with a constant tension. Hence, delicate differences will occur in measurements of assembling accuracy of the mechanism depending on whether the magnetic tape is present or not.
(5) The device is large in size.
Therefore, it was not considered to be a method which would enable accurate measurement of the positions of the components of the assembled mechanism.